Because in electronic parts such as hybrid packages, multimodules, closed type integrated circuits with plastics or metals, etc., with the increase of the integration of integrated circuits, the heating value is increased and there is a possibility of causing functional disorders of electronic parts by the increase of temperature, measures of preventing the occurrence of the functional disorders by attaching heat-radiating members such as a heat sink, etc., to electronic parts are adopted.
For attaching heat-radiating members to electronic parts, a method of fixing by adhesion using an appropriate adhesive may be employed but the following problems are indicated. That is, in the case of using an adhesive prepared by a solution polymerization, a low-boiling organic solvent used at the production remains in the adhesive, and evaporation of the solvent causes corrosion of electronic parts or the evaporation and expansion of the solvent at high temperature cause swelling, peeling, and slipping. Also, in the case of using an adhesive prepared by an emulsion polymerization, an emulsifying agent used at the production thereof bleeds to cause staining, adhesion failure, and lowering of the moisture resistance.
On the other hand, recently, from the view point of safety and the environmental hygiene relating to the use of an organic solvent, photopolymerization-type acrylic pressure-sensitive adhesives are proposed. For example, in U.S. Pat. No. 4,181,752, a method of obtaining a pressure-sensitive adhesive without using an organic solvent, which comprises photopolymerizing an acrylic acid alkyl ester and a monomer for modification on a base material, is disclosed. Also, a pressure-sensitive adhesive prepared by forming the similar pressure-sensitive adhesive on a releasing liner and transferring the adhesive on a base material is known
According to such a photopolymerization-type pressure-sensitive adhesive, it has been expected that the above-described various problems attributable to the use of an organic solvent or an emulsifying agent are all avoided, the molecular weight of the polymer obtained can be increased by irradiating a light having a relatively weak intensity, and a pressure-sensitive adhesive of good heat resistance having a large crosslinking degree and a large cohesive force can be obtained.
Also, the adhesive for attaching heat-radiating members to electronic parts is required to have an excellent heat-conductivity in addition to a high heat resistance.
From this point, for example, (a) an adhesive prepared by adding an aluminum powder to a composition containing a polymerizable acrylic acid ester monomer and a free radical initiator as described in U.S. Pat. No. 4,772,960, (b) a pressure-sensitive adhesive prepared by randomly dispersing a heat-conductive electrically insulating particles in an alkyl acrylate series polymer as described in European Patent (EP) 566093-A1, (c) an adhesive tape having an adhesive layer containing silver particles having the grain sizes larger than the thickness of the layer as described in U.S. Pat. No. 4,606, 962, etc., are proposed.
However, in the case of the adhesive of above-described (a), after applying the adhesive to one or both of an electronic part and a heat-radiating member, it is necessary to apply a curing treatment using a primer or by intercepting oxygen and thus there is a problem that a much time and a large labor are required for the adhesive treatment and also it is required to temporarily fix the adherends until the adhesive is cured, which reduces the production efficiency of electronic parts. Also, in the case of the adhesives, etc., of above-described (b) and (c), when the adhesive used is subjected to a heat cycle of quick heating and quick cooling, peeling and a deterioration are liable to occur and a satisfactory result is not always obtained for fixing by adhesion of electronic parts and heat-radiating members.
The present invention has been made under these circumstances and an object of the present invention is to provide a photopolymerization-type pressure-sensitive adhesive excellent in the heat resistance and the heat conductivity without causing the problems of the adhesive materials added with heat-conductive particles of above-described (a) to (c), that is to provide a pressure-sensitive adhesive excellent in the heat resistance and the heat conductivity and adhesive sheets prepared by forming the pressure-sensitive adhesive in the forms of sheets or tapes, which can easily fix by adhesion electronic parts and heat-radiating members without requiring much time and large labor for the adhering treatment and does not cause the problem of peeling, deterioration, etc., even when the adhesive materials are subjected to a heat cycle of quick heating and quick cooling. As a matter of course, the present invention is not limited to fix well electronic parts and heat-radiating members using the above-described adhesive materials but can give the same effect as above when the adhesive materials are used for fixing other parts or members than the above-described ones.